Very recently, traffics in the Internet are rapidly increased. Under such circumstances, expansions of communication network capacities are strongly required. In connection with these strong demands, wavelength division multiplexing (WDM) transmission techniques have been positively developed and popularized. Since the wavelength division multiplexing transmission techniques correspond to techniques capable of multiplexing a plurality of optical signals having different wavelengths with each other to transmit the multiplexed-optical signal to a single optical fiber, a transmission capacity can be expanded by a total number of multiplexed wavelengths.
To realize a wavelength division multiplexing transmission system, an optical device such as an optical multiplexer/demultiplexer is required. An optical multiplexer/demultiplexer corresponds to such an apparatus for multiplexing, for example, lights having a plurality of wavelengths, and for demultiplexing (separating) multiplexed light to a plurality of light having various wavelengths.
For instance, in such a wavelength division multiplexing transmission system, an optical multiplexer/demultiplexer provided for executing the multiplexing operation multiplexes lights having plural wavelengths with each other. The wavelength-multiplexed light which is formed by the multiplexing operation is transmitted to an optical fiber. Also, for instance, such an optical multiplexer/demultiplexer provided for the demultiplexing operation demultiplexes the wavelength-multiplexed light transmitted via the optical fiber. The demultiplexed light every wavelength is derived.
As one example of such an optical multiplexer/demultiplexer, an optical waveguide type optical multiplexer/demultiplexer may be employed. This optical waveguide type optical multiplexer/demultiplexer is manufactured by that an optical waveguide circuit is formed on a substrate. Since high-precision pattern techniques which have been developed in semiconductor fields may be applied to such an optical waveguide type optical multiplexer/demultiplexer, superior designing characteristics thereof may be obtained.
As this optical waveguide type optical multiplexer/demultiplexer, for example, a Mach-Zehnder interferometer (MZI) type optical multiplexer/demultiplexer has been practically utilized. FIG. 9 indicates a structural example of a circuit (namely, optical multiplexing/demultiplexing circuit) which constitutes such a Mach-Zehnder interferometer type optical multiplexer/demultiplexer.
The optical multiplexing/demultiplexing circuit 8 shown in FIG. 9 includes a first optical waveguide 3, and a second optical waveguide 4 arranged side by side with respect to the first optical waveguide 3. Also, this optical multiplexing/demultiplexing circuit 8 owns a first directional coupling portion 1 formed in such a manner that the first optical waveguide 3 is provided in proximity to the second optical waveguide 4. Also, the optical multiplexing/demultiplexing circuit 8 owns a second directional coupling portion 2 formed in such a manner that the first optical waveguide 3 is provided in proximity to the second optical waveguide 4 at a position via an interval with respect to the first directional coupling portion 1 along a longitudinal direction of the first and second optical waveguides. Both the first optical waveguide 3 and the second optical waveguide 4, which are sandwiched between the adjoining (adjacent) directional coupling portions 1 and 2, own different lengths from each other.
The optical multiplexing/demultiplexing circuit 8 shown in FIG. 9 corresponds to such an optical multiplexing/demultiplexing circuit which may multiplex lights and/or demultiplex light having different wavelengths by properly setting a product (nxc3x97xcex94L), while this product is defined by multiplying a difference xe2x80x9cxcex94Lxe2x80x9d between the length of the first optical waveguide 3 and the length of the second optical waveguide 4, which are sandwiched between both the first directional coupling portion 1 and the second directional coupling portion 2, by diffractive indexes xe2x80x9cnxe2x80x9d of both the first and second optical waveguides 3 and 4.
It should be noted that in the Mach-Zehnder interferometer type optical multiplexing/demultiplexing circuit 8, generally speaking, the below-mentioned path of light is called as a xe2x80x9cthrough transmission path.xe2x80x9d In other words, such a through transmission path corresponds to a path of light which is entered from a light incident side 13 of the first optical waveguide 3 and then is outputted from a light projection side 23 of this first optical waveguide 3, or another path of light which is entered from a light incident side 14 of the second optical waveguide 4 and then is outputted from a light projection side 24 of this second optical waveguide 4. In this specification, a wavelength of such a light designed in such a manner that this light is transmitted via this through transmission path will be referred to as a xe2x80x9cthrough transmission wavelength.xe2x80x9d In FIG. 9, a wavelength xe2x80x9cxcex1xe2x80x9d corresponds to this through transmission wavelength.
Also, in the Mach-Zehnder interferometer type optical multiplexing/demultiplexing circuit 8, generally speaking, the below-mentioned path of light is called as a xe2x80x9ccross transmission path.xe2x80x9d In other word, such a cross transmission path corresponds to a path of light which is entered from the light incident side 13 of the first optical waveguide 3 and then is outputted from the light projection side 24 of the second optical waveguide 4, or another path of light which is entered from the light incident side 14 of the second optical waveguide 4 and then is outputted from the light projection side 23 of the first optical waveguide 3. In this specification, a wavelength of such a light designed in such a manner that the light is transmitted via this cross transmission path will be referred to as a xe2x80x9ccross transmission wavelength.xe2x80x9d In FIG. 9, a wavelength xe2x80x9cxcex2xe2x80x9d corresponds to this cross transmission wavelength.
Also, for example, as shown in FIG. 10, such an optical multiplexer/demultiplexer has been proposed which is formed by connecting Mach-Zehnder interferometer type optical multiplexing/demultiplexing circuits 8 (8A to 8G) to each other in a tree shape. This optical multiplexer/demultiplexer contains plural stages of optical multiplexing/demultiplexing circuit 8, while plural stages (three stages in this example) are defined from a first stage up to an M-th stage (symbol xe2x80x9cMxe2x80x9d being integer larger than, or equal to 2, namely, 3 is selected in this example). That is, the plural stages of the optical multiplexing/demultiplexing circuits 8 are formed by arranging one, or plural sets of optical multiplexing/demultiplexing circuits 8 (8A to 8G) side by side.
In the optical multiplexer/demultiplexer shown in FIG. 10, for instance, a first stage of plural optical multiplexing/demultiplexing circuits 8 (8A, 8B, 8C, 8D) multiplex light entered from a corresponding first optical waveguide 3 and light entered from a corresponding second optical waveguide 4 respectively, and then, output the multiplexed light from either the first optical waveguide 3 or the second optical waveguide 4 respectively. A second stage of an optical multiplexing/demultiplexing circuit 8(8E) furthermore multiplexes the optical outputs from the first stage of one pair of optical multiplexing/demultiplexing circuits 8 (8A, 8B). The second stage of this optical multiplexing/demultiplexing circuit 8 (8F) furthermore multiplexes the optical outputs from the first stage of one pair of optical multiplexing/demultiplexing circuits 8 (8C, 8D).
Furthermore, a third stage of optical multiplexing/demultiplexing circuit 8 (8G) multiplexes the optical outputs from the second stage of one pair of optical multiplexing/demultiplexing circuits 8 (8E, 8F). As previously explained, in the optical multiplexer/demultiplexer shown in FIG. 10, the post stage of the optical multiplexing/demultiplexing circuits 8 furthermore multiplex the optical outputs from the pre-stage of one pair of the optical multiplexing/demultiplexing circuits 8.
In the optical multiplexer/demultiplexer shown in FIG. 10, the first stage of the optical multiplexing/demultiplexing circuit 8A multiplexes light having a wavelength of xe2x80x9cxcexaxe2x80x9d and light having a wavelength of xe2x80x9cxcexbxe2x80x9d, and also the first stage of the optical multiplexing/demultiplexing circuit 8B multiplexes light having wavelength of xe2x80x9cxcexcxe2x80x9d and light having a wavelength of xe2x80x9cxcexd.xe2x80x9d Also, the first stage of the optical multiplexing/demultiplexing circuit 8C multiplexes light having a wavelength of xe2x80x9cxcexexe2x80x9d and light having a wavelength of xe2x80x9cxcexfxe2x80x9d, and also the first stage of the optical multiplexing/demultiplexing circuit 8D multiplexes light having a wavelength of xe2x80x9cxcexgxe2x80x9d and light having a wavelength of xe2x80x9cxcexh.xe2x80x9d
Further, the second stage of the optical multiplexing/demultiplexing circuit 8E multiplexes the lights having the wavelengths of xe2x80x9cxcexaxe2x80x9d, xe2x80x9cxcexbxe2x80x9d, xe2x80x9cxcexcxe2x80x9d, xe2x80x9cxcexdxe2x80x9d with each other, and also, the second stage of the optical multiplexing/demultiplexing circuit 8F multiplexes the lights having the wavelengths of xe2x80x9cxcexexe2x80x9d, xe2x80x9cxcexfxe2x80x9d, xe2x80x9cxcexgxe2x80x9d, xe2x80x9cxcexhxe2x80x9d with each other. In addition, the third stage of the optical multiplexing/demultiplexing circuit 8G multiplexes the lights having the wavelengths of xe2x80x9cxcexaxe2x80x9d, xe2x80x9cxcexbxe2x80x9d, xe2x80x9cxcexcxe2x80x9d, xe2x80x9cxcexdxe2x80x9d, xe2x80x9cxcexexe2x80x9d, xe2x80x9cxcexfxe2x80x9d, xe2x80x9cxcexgxe2x80x9d, and xe2x80x9cxcexhxe2x80x9d with each other, and then the multiplexed light is outputted from the second optical waveguide 4 of the optical multiplexing/demultiplexing circuit 8.
It should be noted that the optical multiplexer/demultiplexer indicated in FIG. 10 owns a reciprocity characteristic of an optical circuit. As a result, contrary to FIG. 10, in such a case that the multiplexed light having the wavelengths of xe2x80x9cxcexaxe2x80x9d, xe2x80x9cxcexbxe2x80x9d, xe2x80x9cxcexcxe2x80x9d, xe2x80x9cxcexdxe2x80x9d, xe2x80x9cxcexexe2x80x9d, xe2x80x9cxcexfxe2x80x9d, xe2x80x9cxcexgxe2x80x9d, and xe2x80x9cxcexhxe2x80x9d is inputted from the second optical waveguide 4 of the optical multiplexing/demultiplexing circuit 8 (8G), this multiplexed light is demultiplexed by these optical multiplexing/demultiplexing circuits 8 (8A to 8G) to produce a plurality of light having the respective wavelengths. Then, a plurality of demultiplexed lights are outputted from the first stage of the plural optical multiplexing/demultiplexing circuits 8 (8A, 8B, 8C, 8D) respectively.
An optical multiplexer/demultiplexer, according to the present invention, is featured by comprising:
optical multiplexing/demultiplexing circuits of Mach-Zehnder optical interferometers which are connected in plural stages; wherein:
the optical multiplexing/demultiplexing circuit of one Mach-Zehnder optical interferometer is comprised of:
a first optical waveguide;
a second optical waveguide arranged side by side with respect to the first optical waveguide; and
a first directional coupling portion formed in such a manner that the first optical waveguide is provided in proximity to the second optical waveguide; and a second directional coupling portion formed in such a manner that the first optical waveguide is provided in proximity to the second optical waveguide at a position with an interval with respect to the first directional coupling portion along a longitudinal direction of the first and second optical waveguides;
both the first optical waveguide and the second optical waveguide, which are sandwiched between both the first directional coupling portion and the second directional coupling portion, own different lengths from each other; and wherein:
in each of the plural stages of the optical multiplexing/demultiplexing circuits, in such a case that either a wavelength of such light which is inputted from a light incident side of the first optical waveguide and then is outputted from a light projection side of the second optical waveguide, or another wavelength of such light which is inputted from a light incident side of the second optical waveguide and then is outputted from a light projection side of the first optical waveguide is defined as a cross transmission wavelength; and
in such a case that either a wavelength of such light which is inputted from the light incident side of the first optical waveguide and then is outputted from the light projection side of the first optical waveguide, or another wavelength of such light which is inputted from the light incident side of the second optical waveguide and then is outputted form the light projection side of the second optical waveguide is defined as a through transmission wavelength;
an averaged wavelength of the cross transmission wavelengths is made longer than an averaged wavelength of the through transmission wavelengths in the respective stages of the optical multiplexing/demultiplexing circuits.